The Access Code says:
You can avoid disturbing sheep close to lambing time, or young animals such as calves, lambs, foals and farmed deer, by going into a neighbouring field or onto adjacent land. If this is not possible, keep as far from the animals as possible. Do not take dogs into fields where there are young animals present.
Specific advice for dog walkers can be found in the Practical Guide for All on the dog walking pages.